【TED-Ed】宇宙尘埃 | The dust bunnies that built our planet

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2020年2月5日16:19:00【TED-Ed】宇宙尘埃 | The dust bunnies that built our planet已关闭评论 242 次浏览
【TED-Ed】宇宙尘埃 | The dust bunnies that built our planet

考虑你现在所坐的位置。时光倒流,它可能被淹没在浅海底部,埋在数英里深的岩石下,或者漂浮在熔融的景观中。但是回到46亿年前,你会被淹没在中间环绕着一颗新生恒星的一个巨大尘埃云和气体云中。这个宇宙尘埃到底是什么?洛林·马修斯(Lorin Matthews)为我们探究了一番。

Consider the spot where you’re sitting. Travel backwards in time and it might’ve been submerged at the bottom of a shallow sea, buried under miles of rock, or floating through a molten, infernal landscape. But go back far enough— about 4.6 billion years, and you’d be in the middle of an enormous cloud of dust and gas orbiting a newborn star. This is the setting for some of the biggest, smallest mysteries of physics: the mysteries of cosmic dust bunnies.

考虑你现在坐的地方,时光倒流,它可能被淹没在浅海底部,埋在几英里深的岩石下,或者漂浮在熔融的景观中。但是回到久远的46亿年前,你将置身于无边的尘埃和气体云团中,围绕一个新的星球旋转。这是物理学最大,也是最小的谜团:宇宙尘埃的谜团。

Seemingly empty regions of space between stars actually contain clouds of gas and dust, usually blown there by supernovas. When a dense cloud reaches a certain threshold called the Jeans mass, it collapses in on itself. The shrinking cloud rotates faster and faster, and heats up, eventually becoming hot enough to burn hydrogen in its core. At this point a star is born. As fusion begins in the new star, it sends out jets of gas that blow off the top and bottom of the cloud, leaving behind an orbiting ring of gas and dust called a protoplanetary disk. This is a surprisingly windy place; eddies of gas carry particles apart, and send them smashing into each other. The dust consists of tiny metal fragments, bits of rock, and, further out, ices.

看似空旷的星际空间实际上存在着气体和尘埃云团,通常被超新星吹到那里。当云团密度达到临界值,即金斯质量,它自身会坍塌。收缩的云团越转越快,逐渐升温,最后在内核点燃氢气。一个新的星球在此刻诞生。随着新的星球开始出现聚变反应,它喷发出的气体将云团顶部和底部吹散,留下一个环绕星球旋转的气体和尘埃环,被称为原行星盘。这个地方的风出奇的大;气流旋涡带动粒子分离,使它们互相碰撞,宇宙尘埃由小的金属碎片,岩石颗粒,以及冰块组成。

We’ve observed thousands of these disks in the sky, at various stages of development as dust clumps together into larger and larger masses.

我们可以在天空中观察到数千个类似的原行星盘,它们处于不同的发展阶段,随着尘埃聚集成群,变成更大的云团。

Dust grains 100 times smaller than the width of a human hair stick to each other through what’s called the van der Waals force. That’s where a cloud of electrons shifts to one side of a molecule, creating a negative charge on one end, and a positive charge on the other. Opposites attract, but van der Waals can only hold tiny things together.

尘埃颗粒比人类的一根头发丝还小100倍,它们之间的引力被称为范德华力。那里电子云团会转移到分子的一侧,使得分子的一端产生正电荷,一端产生负电荷。正负相吸,范德华力将它们吸引在一起。

And there’s a problem: once dust clusters grow to a certain size, the windy atmosphere of a disk should constantly break them up as they crash into each other. The question of how they continue to grow is the first mystery of dust bunnies.

但问题是:一旦尘埃团增长到一定规模,星盘的多风空气层总是会把它们打碎,它们随之相互碰撞。宇宙尘埃的一大谜题就是它们如何继续膨胀。

One theory looks to electrostatic charge to answer this. Energetic gamma rays, x-rays, and UV photons knock electrons off of gas atoms within the disk, creating positive ions and negative electrons. Electrons run into and stick to dust, making it negatively charged. Now, when the wind pushes clusters together, like repels like and slows them down as they collide. With gentle collisions they won’t fragment, but if the repulsion is too strong, they’ll never grow. One theory suggests that high energy particles can knock more electrons off of some dust clumps, leaving them positively charged. Opposites again attract, and clusters grow rapidly.

静电力理论也许能回答这个问题。高能伽马射线,X射线,紫外线光子使星盘中气体原子携带的电子减少,从而制造出正离子和负电子。电子与尘埃的结合使尘埃携带负电荷,当风把尘埃粒子聚在一起,同性相斥,碰撞也使得粒子的运动速度降低。轻微的碰撞不会使它们碎裂,但是如果相斥力太强,它们就没法长大。有一种理论说的是,高能量粒子可以使尘埃团的电子减少,使得它们携带正电荷。异性相吸,云团迅速增长.

But before long we reach another set of mysteries. We know from evidence found in meteorites that these fluffy dust bunnies eventually get heated, melted and then cooled into solid pellets called chondrules. And we have no idea how or why that happens. Furthermore, once those pellets do form, how do they stick together? The electrostatic forces from before are too weak, and small rocks can’t be held together by gravity either. Gravity increases proportionally to the mass of the objects involved. That’s why you could effortlessly escape an asteroid the size of a small mountain using just the force generated by your legs. So if not gravity, then what? Perhaps it’s dust. A fluffy dust rim collected around the outside of the pellets could act like Velcro. There’s evidence for this in meteors, where we find many chondrules surrounded by a thin rim of very fine material– possibly condensed dust.

但不久我们就会发现另一组谜团。我们从陨石中找到了证据,证明这些蓬松的尘埃团最终被加热,融化,之后冷却成固体的陨石颗粒。但我们并不知道这一过程究竟是如何发生的。一旦颗粒团形成,它们又是如何黏在一起的呢?之前的静电力很弱,小的石块不会在引力下结合在一起,万有引力随着物体的质量变大按比例地增加。这就是为什么仅仅使用脚上的力量,我们就可以毫不费力地从一个像小山一样大的小行星中逃逸。那么如果不是引力,又是什么?可能是尘埃。在颗粒团的外部收集的蓬松的灰尘边缘可以像尼龙搭扣一样运动。在陨石中可以找到相关的证据,我们发现陨石颗粒上面有一层薄薄的,质地细密的矿物质——可能是凝结的尘埃。

Eventually the chondrule pellets get cemented together inside larger rocks, which at about 1 kilometer across are finally large enough to hold themselves together through gravity. They continue to collide and grow into larger and larger bodies, including the planets we know today.

最后陨石颗粒粘合在一起形成更大的岩石,直径达1公里,最终大到足够在引力下支撑自己。它们继续撞击并增长到更大的体积,包括我们已知的行星。

Ultimately, the seeds of everything familiar– the size of our planet, its position within the solar system, and its elemental composition– were determined by an uncountably large series of random collisions. Change the dust cloud just a bit, and perhaps the conditions wouldn’t have been right for the formation of life on our planet.

最终,所有的事情开始变得熟悉——行星的规模,它们在太阳系的位置,以及它们的元素组成——是由无数随机的碰撞形成的。稍微改变一下尘埃云,也许我们星球上的环境就不再适合生命的形成。

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